1. Field of the Invention
The invention relates to device for absorbing shock upon the fall of neutron absorbing bars in a nuclear reactor fuel assembly cooled by a substantially incompressible fluid, such as a liquid, of the type including a piston and a cylinder, providing a restricted fluid leak path out of the cylinder when the piston is driven into the cylinder from a rest position into which the piston is urged by spring means.
The invention applies more particularly to reactors whose absorbing bars each include a cluster of parallel neutron absorbing rods fixed to arms of a spider having a central pommel or hub connectable to a driving mechanism for vertical movement. It is particularly suitable for use in reactors cooled and moderated by pressurized water (PWRs).
Neutron absorbing bars of the above kind, whose rods contain a neutron poison, are used for controlling the reactivity in the core of the reactor, by inserting the rods to a greater or lesser extent. To cause an emergency shut down of the reactor, all control bars are simultaneously dropped into the core by releasing them so that they are driven into the core by their own weight.
2. Prior Art
If has already been proposed to provide a neutron absorbing bar with a shock absorber to limit the load when the pommel comes into abutment against the upper core plate of the reactor or against the upper end piece of the fuel assembly with which it is associated.
European 159,059 describes a shock absorbing device of the above-defined type having a cylinder formed in the pommel. A piston slidably received in the cylinder is urged into a downward projecting position by resilient means contained in the cylinder. From the moment when the piston comes into abutment against the upper core plate, downward movement is braked by the compression of the resilient means and by the head loss undergone by the liquid which flows out of the cylinder between the wall thereof and the piston.
Such a shock absorbing device is of limited efficiency. The shock absorbing effect provided by the head loss undergoes practically no change during movement of the piston and only the increasing stiffness of the spring ensures progressivity. Since the device is integrated in the pommel, it interferes with liquid flow and cooling. The device is not easily removable for replacement or re-use on another cluster.
Another arrangement which is widely used for absorbing the load upon falling of the bars uses the head-loss imparted to the coolant flow driven by the rods out of the guide tubes during the fall of the bar. For increasing the head loss, a restriction is formed at the lower part of the tubes: that approach involves troublesome constraints as far as the construction of the guide tubes is concerned and it increases their cost.